By its hatching date. A veterinarian may be able to estimate the animal's age based on shell condition, overall size, etc, but there's no way to pinpoint it. (No, you can't tell a tortoise's age by counting the growth rings on its scutes - those only denote periods of faster growth.
Correspondingly, how big do desert tortoise get?
One may also ask, how do you tell the difference between a male and female desert tortoise?
A male tortoise has a longer gular horn than a female, his plastron (lower shell) is concave compared to a female tortoise. Males have larger tails than females do. Their shells are high-domed, and greenish-tan to dark brown in color. Desert tortoises can grow to 4–6 in (10–15 cm) in height.
How long does a desert tortoise live?
How old is a turtle in human years?